1. Field of the Invention
The present invention relates to electric power tools, such as grinders, that transmit rotation of a motor via the motor shaft to a gear disposed coaxially with a spindle. More particularly, the present invention relates to an electric power tool such as described above that includes a mechanism for reducing impact on the tool during operation.
2. Description of the Related Art
A certain type of grinder includes a motor shaft having a gear at its free end and a spindle disposed orthogonally to the motor shaft in a housing. Another gear, such as a bevel gear, is secured to the spindle by suitable means, such as key-groove coupling or press fitting. The bevel gear on the spindle meshes with the gear of the motor shaft so as to transmit torque from the motor to spindle via the bevel gear. The grinding additionally includes a recess in a portion of the bevel gear where the teeth are not formed and a locking device that is biased in the direction away from the recess in the bevel gear. The locking device can be manually pushed into the recess for locking the spindle via the bevel gear in order to facilitate tightening and loosening of a lock nut that attaches, for example, a grinding wheel to the spindle.
When a locking device is used as a brake in a grinder with no electric brake, the tool bit, such as a grinding wheel, continues to rotate due to its own inertia even after the motor is turned off. Thus, if the operator wishes to quickly change the tool bit for a different task, the locking device is used as a brake by bringing the device into contact with the bevel gear. This, however, subjects not only the locking device but also the bevel gear, the spindle, and the housing to severe impact, which may cause deformation or breakage of these elements.
Moreover, as the lock nut tends to be tightened by the rotation of the grinder wheel, when the motor is started, the lock nut is subjected to an impact acting in the direction in which the nut is tightened. When electric brakes (if provided) are applied or the locking device is used as a brake during rotation of the tool bit due to its own inertia, a difference in rotational speed develops between the tool bit, which tends to stay in rotation due to its inertia, and the decelerating bevel gear and spindle. As this difference in speed generates an impact on the lock nut acting in the direction opposite to that in which the lock nut is tightened, the lock nut may be inadvertently loosened.